REGULATORY MATTERS, COMMITMENTS AND CONTINGENCIES - Regulatory capital positions (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Securitization of FHA/VA mortgage loan production into GNMA mortgage-backed securities	$ 235,100
To originate loans [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	77,649	$ 41,271
Unused personal lines of credit [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	710,607	667,552
Commercial lines of credit [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	471,732	421,437
Commercial Letters Of Credit [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	46,728	47,515
Standby letters of credit [Member] [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	2,691	2,556
Commitments To Sell Loans [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	37,909	119,679
First Ban Corp [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital	1,989,873	1,921,329
Common Equity Tier 1 Capital To Risk Weight Assets	1,674,164	1,597,117
Tier 1 Capital	1,675,282	1,597,117
Tier 1 Leverage Capital	1,675,282	1,597,117
Total Capital Required For Capital Adequacy	706,432	720,329
Common Equity Tier 1 Capital To Risk Weight Assets Capital Adequacy	397,368	405,185
Tier 1 Risk Based Capital Required for Capital Adequacy	529,824	540,247
Tier 1 Leverage Capital Required for Capital Adequacy	$ 477,643	$ 466,376
Total Risk Based Capital Ratio	22.53%	21.34%
Common Equity Tier 1 Capital To Risk Weight Assets Ratio	18.96%	17.74%
Tier 1 Risk Based Capital Ratio	18.97%	17.74%
Tier 1 Leverage Ratio	14.03%	13.70%
Total Risk Based Capital Ratio Adequately Capitalized	8.00%	8.00%
Common Equity Tier 1 Capital To Risk Weight Assets Ratio Capital Adequacy	4.50%	4.50%
Tier 1 Risk Based Capital Ratio Adequately Capitalized	6.00%	6.00%
Tier 1 Leverage Ratio Adequately Capitalized	4.00%	4.00%
FirstBank [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital	$ 1,947,627	$ 1,872,120
Common Equity Tier 1 Capital To Risk Weight Assets	1,562,431	1,523,332
Tier 1 Capital	1,835,445	1,757,642
Tier 1 Leverage Capital	1,835,445	1,757,642
Total Capital Required For Capital Adequacy	706,218	720,091
Common Equity Tier 1 Capital To Risk Weight Assets Capital Adequacy	397,248	405,051
Tier 1 Risk Based Capital Required for Capital Adequacy	529,663	540,068
Tier 1 Leverage Capital Required for Capital Adequacy	477,056	465,740
Total Capital Required to be Well Capitalized	882,772	900,114
Common Equity Tier 1 Capital To Risk Weight Assets Well Capitalized	573,802	585,074
Tier 1 Risk Based Capital Required to be Well Capitalized	706,218	720,091
Tier 1 Leverage Capital Required to be Well Capitalized	$ 596,320	$ 582,174
Total Risk Based Capital Ratio	22.06%	20.80%
Common Equity Tier 1 Capital To Risk Weight Assets Ratio	17.70%	16.92%
Tier 1 Risk Based Capital Ratio	20.79%	19.53%
Tier 1 Leverage Ratio	15.39%	15.10%
Total Risk Based Capital Ratio Adequately Capitalized	8.00%	8.00%
Common Equity Tier 1 Capital To Risk Weight Assets Ratio Capital Adequacy	4.50%	4.50%
Tier 1 Risk Based Capital Ratio Adequately Capitalized	6.00%	6.00%
Tier 1 Leverage Ratio Adequately Capitalized	4.00%	4.00%
Total Risk Based Capital Ratio Well Capitalized	10.00%	10.00%
Common Equity Tier1 Capital To Risk Weight Assets Ratio Well Capitalized	6.50%	6.50%
Tier 1 Risk Based Capital to Risk Weighted Well Capitalized	8.00%	8.00%
Tier 1 Leverage Ratio Well Capitalized	5.00%	5.00%

Common Equity Tier 1 capital as defined by new Basel III rule.

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Common Equity Tier 1 capital divided by risk weighted assets for capital adequacy as defined by new Basel III rule.

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Common Equity Tier 1 capital divided by risk weighted assets as defined by new Basel III rule.

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Minimun common equity tier 1 capital to risk weight assets for well capitalized purposes plus capital conservation buffer as required by Basel III rule.

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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Represents the initial fair value recorded for liabilities incurred on transfer of financial assets in a securitization, asset-backed financing arrangement, or a similar transfer which transaction is recognized as a sale of the transferred financial assets.

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